Energy-Efficient Desalination Solutions for Low-Emission Cruise Line Vessels

Introduction

With the final commissioning completed in 2023, Salt Separation Services completed the delivery of six Seawater Reverse Osmosis (SWRO) plants for two cruise ships (three SWRO plants per ships). Each SWRO plant has a capacity of 3 x 300 m³/d. These plants were designed for two modern, low-emission luxury passenger vessels, emphasising environmentally friendly cruising. 

Challenges

The cruise builder’s commitment to low emissions necessitated a highly energy-efficient SWRO plant solution. Advances in RO technology, particularly in Energy Recovery Devices (ERDs), have significantly improved the Specific Energy Consumption (SEC) of such systems. While typical SEC values for marine and cruise SWRO desalination range from 3.5 to 5 kWh/m³, un-optimised plants lacking ERDs and other energy-saving measures can easily exceed 7.5 kWh/m³. 

Salt Sep were tasked with designing and delivering SWRO desalination plants with an SEC of less that 3.5kWh/m³, each capable of producing 300 m³/d. Beyond this ambitious energy efficiency target, the customer required RO permeate of the highest technical quality, with a chloride concentration of no more than 15 mg/l from seawater with a salinity of 36,000 mg/l at a temperature of 32°C. Adding to the challenge, the plant design needed to fit within a compact space envelope of 8m x 2.8m x 2m and operate efficiently with feedwater temperatures as low as -2°C. 

Solution and Innovation

To address the challenge, Salt Sep implemented Danfoss Axial Piston Pumps in conjunction with Danfoss iSave ERDs achieving exceptionally low SEC. To further improve efficiency, Danfoss Variable Frequency Drives (VFDs) were incorporated, allowing precise control over the electrical motors. Without VFDs, the motors would operate at maximum electrical load regardless of actual demand, resulting in unnecessary energy consumption. By dynamically adjusting motor loads, the use of VFDs contributed to significant additional energy savings. 

In addition to equipment selection, the system utilised the latest generation of DuPont FilmTec membranes, which further optimised SEC. These advanced spiral-wound membranes were chosen for their superior performance, high recovery rates, resistance to fouling and low energy characteristics. Their proven reliability and a three-year limited prorated element warranty from DuPont added value and confidence to the design. 

To meet the stringent RO permeate quality specified by the customer, the plant was configured as a two-pass system. In this design, permeate from the first pass of RO vessels is fed into the second pass, producing high-quality technical water. This innovative approach included a single high-pressure pump sized to drive both passes, offering multiple benefits such as reduced equipment maintenance and minimised spatial requirements. The result was a compact, efficient system that delivered exceptional performance while adhering to the customer’s technical and operational constraints. Extensive modelling and simulation revealed that this solution was more energy efficient than using a separate second pass high pressure multistage centrifugal pump.

To learn more about how Danfoss’ innovative solutions supported this project, visit their detailed case study here.

Result

We successfully delivered SWRO desalination plants with a projected SEC ranging from 3.04 to 3.36 kWh/m³, depending on feedwater conditions. Each plant is designed to produce 300 m³/d from seawater, operating efficiency across a wide temperature range, from sub-freezing conditions to 32°C. 

To ensure optimal performance, each system includes comprehensive pre-treatment features, such as appropriately sized media filters and cartridge filters, to protect the pumps and condition the feedwater before it enters the RO stack. Additionally, a titanium plated heat exchanger using waste heat is integrated into the skid to maintain feedwater temperatures at a minimum of 5°C, ensuring reliable operational under varying conditions. 

For streamlined operation and maintenance, a single localised electrical panel is mounted within the skid, housing all necessary controls and logic. 

Since 1990, Salt Separation Services has been a trusted provider of RO equipment and services to cruise liners, naval vessels, offshore platforms, onshore industries and more, worldwide. Our SWRO desalination plants are designed to meet specific customer needs, incorporating the latest advancements in technology to ensure state-of-the-art performance. For more information or to discuss your requirements, please don’t hesitate to get in touch with us – sss@saltsep.co.uk.